The storage density of a semiconductor memory device represented by a DRAM (Dynamic Random Access Memory) is increasing year by year due to advancement in a microfabrication technique. Along with the progress in downsizing, the number of defective memory cells included per one chip is also increasing. Generally, such defective memory cell is replaced by a redundant memory cell, thereby relieving a defective address.
Generally, the defective address is stored in a program circuit including a plurality of fuse elements. When the defective address is accessed, the program circuit detects the access request. As a result, an alternate access is performed not to the defective memory cell but to the redundant memory cell. As the configuration of the program circuit, as described in Japanese Patent Application Laid Open No. H9-69299, there is known a method of storing a desired address by allocating a pair (two) of fuse elements to each bit that constitutes addresses to be stored, and cutting one of the two fuse elements.
There is also known a method of allocating one fuse element to each bit that constitutes addresses to be stored, as described in Japanese Patent Application Laid Open No. H6-119796. In this method, it is possible to store one bit by whether to cut the one fuse element. Thus, it becomes possible to greatly reduce the number of fuse elements.
As a method of cutting a fuse element, there are known, roughly, two methods. One method is to fuse the fuse element with a high current (see Japanese Patent Application Laid Open Nos. 2005-136060 and 2003-501835). The other method is to destruct the fuse element by irradiation with a laser beam (see Japanese Patent Application Laid Open Nos. H7-74254 and H9-36234). The former method is advantageous in that an expensive device such as a laser trimmer is not required, and whether the fuse element is correctly cut can be easily self-evaluated. However, to use this method, a fuse cutting circuit and a diagnostic circuit need to be employed inside the semiconductor device, which increases the chip area.
In contrast, in the method of destructing the fuse element by laser beam irradiation, the fuse cutting circuit or the like need not be employed inside the semiconductor device. Accordingly, it is possible to reduce the chip area. However, in this method, a passivation film is also destructed by laser beam irradiation. As a result, moisture enters from the destructed area, which often becomes a cause of a decrease in reliability of the semiconductor device.
In the method of destructing the fuse element by laser beam irradiation, materials of the destructed passivation film, the fuse element, and the like scatter, and debris adheres to an objective lens that converges the laser beam. To prevent the adhesion of the debris, an optical system having a relatively large focal distance can be used to provide a distance between the objective lens and the semiconductor device. In this optical system, however, inevitably, it becomes necessary to make a numerical aperture (NA) of the objective lens small. As a result, the depth of focus becomes large, so that a high density energy is applied not only to the fuse element to be cut but also to members located above or below the fuse element. This makes it impossible to arrange a wiring and a transistor immediately above or below the fuse element.
Further, holes extending in a crater shape are formed on the passivation film irradiated with the laser beam. To prevent the crater or a crack caused thereby from affecting other fuse elements, it is necessary to arrange adjacent fuse elements with a sufficient distance provided therebetween. This makes it difficult to enhance an arrangement density of the fuse elements.